I need help with these 3 questions

1) Carly is training by running to a post across a field and then back. She runs the outward leg at 7m/s and the return leg at 5m/s. She takes 15.4s.
Find the distance to the post.

How do I solve, using algebra, the following?

Please don't tell me the answers, just give me some pointers as I have just been staring at these questions for about half an hour unsure what to do.

The questions before were really easy like solve 5x -3 = 17 etc , so I have tried to solve them in the same way I would solve the easy questions, but it hasn't worked at all.

1) Carly is training by running to a post across a field and then back. She runs the outward leg at 7m/s and the return leg at 5m/s. She takes 15.4s.
Find the distance to the post.

2) A piece of wire 30cm long is cut into 2 pieces. One of these is bent into a circle, and the other is bent into a square enclosing the circle.
what is the diameter of the cirlce?

3) Sophie sets off on a walk at 6km/h. 10 minutes later, her brothers Dave sets off after her on his bicycle at 15km/h. How far must Dave go to catch up with Sophie?

You need to use $\text{time}=\frac{\text{distance}}{\text{speed}}$.

time1 = d/7

time2 = d/5

time1+time2 = 15.4.

Can you form your equation from there?

2. let diameter of circle be d.
What is its circumference in terms of d ?
what is the perimeter of the square ?
add together and put equal to 30.

3. Let distance before Dave catches Sophie be x km.
How long has it taken Sophie to travel this far?
How long does it take Dave?
Dave's time must be 10 mins less than Sophies
Be careful with units. Don't mix hours and minutes.